This invention relates to an image-forming system containing spatial light modulators that generate a visible image from an electronic signal. More particularly, the invention relates to a projection display system having enhanced gray levels for each pixel in the image.
Many display systems, including projection and video type display systems, employ a uniformly illuminated spatial light modulator to convert electronic image information into an output image. At present in such systems, the light source is typically a white light lamp and the spatial light modulator is typically an area array containing liquid crystal devices or micromirror devices. Alternative display system architectures with one or more laser sources and spatial light modulators that are linear arrays of electromechanical grating devices show much promise for the future. To display high quality motion images, the individual devices of these different spatial light modulators must be capable of rapidly producing a large number of gray levels in the image. The limit on the number of possible gray levels is usually dictated either by the device dynamics or by the speed of electronics.
Prior inventions have disclosed schemes for increasing the number of gray-levels in the image without increasing the speed of the modulating elements or of the associated electronics. These schemes vary the illumination that is incident on the spatial light modulator during a frame. Specifically, according to U.S. Pat. No. 5,812,303, issued to Hewlett et al. on Sep. 22, 1998, entitled, xe2x80x9cLIGHT AMPLITUDE MODULATION WITH NEUTRAL DENSITY FILTERS,xe2x80x9d additional gray levels can be obtained with a micromirror device by using a variable neutral density filter to generate coarse and fine gray levels. The fine gray scale is obtained by attenuating the illumination for some time during the display of a frame. The coarse gray scale has no attenuation. In practice, the invention is implemented by rotating a filter wheel with a multi-segment neutral density filter in synchronization with the data stream.
An alternative approach employs a pulsating light source such as a pulsed laser to reduce speed requirements on the electronics, as described in U.S. Pat. No. 5,668,611, issued to Ernstoff et al. on Sep. 16, 1997, entitled xe2x80x9cFULL COLOR SEQUENTIAL IMAGE PROJECTION SYSTEM INCORPORATING PULSE RATE MODULATED ILLUMINATION.xe2x80x9d The illumination on the spatial light modulator is adjusted by varying the pulse rate or pulse count. Moreover, the average brightness of the light source is determined by the number of pulses. A complementary method uses direct intensity modulation of the light source to obtain multiple levels of brightness, as disclosed in U.S. Pat. No. 5,903,323, issued to Ernstoff et al. on May 11, 1999, entitled xe2x80x9cFULL COLOR SEQUENTIAL IMAGE PROJECTION SYSTEM INCORPORATING TIME MODULATED ILLUMINATION.xe2x80x9d Both U.S. Pat. No. 5,668,611 and U.S. Pat. No. 5,903,323 address the specific problem of having a large enough time window for the electronics to load new image data bits into the spatial modulator.
Each of the above described inventions trade light source efficiency for improved gray levels or reduced electronic speed requirements. However, efficient use of the light source is needed for theater-type projection displays in order to maximize brightness and color saturation of the projected image.
The display systems described in the prior art achieve fine gray level control by lowering the average optical power incident on the spatial light modulator for some period of time, thus generating multiple illumination levels corresponding to decreased intensity. Multiple illumination levels reduce the speed requirements of the spatial light modulator and its associated electronics. However, a serious technical drawback to this approach is that it wastes optical power that is available from the light source during lower illumination intervals. There is a need, therefore, for a display system having fine gray level control while simultaneously making efficient use of available optical power.
The above need is met according to the present invention by providing an image-forming system with enhanced gray levels that includes a first light source that can emit a primary beam of light having a primary intensity value, a second light source that can emit a secondary beam of light having a secondary intensity value significantly less than the primary intensity value, a first modulator array of discrete devices receiving the primary beam of light and producing an output with coarse gray levels, a second modulator array of discrete devices receiving the secondary beam of light and producing an output with fine gray levels; a controller for synchronously controlling the first and the second modulator array, and optics that combine the output with fine gray levels and the output with coarse gray levels to form an image with the enhanced gray levels.